In Chapter 6 of the Apokalypsis each of them is broken/opened by "the Lamb," who is referred to in the previous chapter as the Lion of the Tribe of Judah, and the Root of David.
In the first 7 verses of Chapter 5: I saw a Book on the right of the Enthroned One, written inside and on the back, sealed with seven seals. And I saw a mighty angel proclaiming with a loud voice, "Who is worthy to open the Book and to loosen its seals?" And no one in Heaven nor on the Earth nor under the Earth was able to open the Book or to look at it. And I wept very much, because no one was found worthy to open and to read the Book, nor to look at it. And one of the Elders said to me, "Do not weep. Behold, the Lion of the Tribe of Judah, the Root of David, has prevailed to open the Book and to loose its seven seals." And I looked, and behold, in the midst of the Throne and of the four living creatures, amidst the Elders, a Lamb stood, as if it had been slain, having seven horns and seven eyes, which are the seven Spirits of God sent forth into all the earth. And He came and took the Book out of the right hand of the Enthroned One.
And then in Chapter 6 the Lamb goes on to open each seal, one by one, each time with a different result either on earth or in the heavens.